Method for producing self-cleaning ceramic layers and a composition therefore

ABSTRACT

The invention concerns a method for producing a highly-porous ceramic layer and for application of this layer onto metallic, ceramic, enameled and/or glass substrates using porous, ceramic particles, preferably aluminum oxide, titanium oxide and zircon oxide, and an inorganic binder system. The inorganic binder system contains at least one ceramic nanoparticle of a particle size of less than 100 nm, preferably less than 50 nm and particularly preferred less than 25 nm, the solvent being water. Layers produced in this fashion are suited as self-cleaning catalytically active layers e.g. in ovens, in combustion engines etc. or for general coating of substances to considerably increase their specific surface e.g. for catalytic substrates.

BACKGROUND OF THE INVENTION

The invention concerns a method for producing a highly-porous, ceramiclayer which can be applied to metallic, ceramic, enameled and/or glasssubstrates using porous, ceramic particles, preferably aluminum oxide,titanium oxide and zirconium oxide and an inorganic binder system. Theinorganic binder system contains at least one ceramic nanoparticle of aparticle size of less than 100 nm, preferably less than 50 nm andparticularly preferred less than 25 nm, the solvent being water. Layersproduced in this fashion are suited for self-cleaning catalyticallyactive layers e.g. in ovens, in combustion engines etc. or for generalcoating of substrates, to considerably increase their specific surfacearea e.g. for catalytic applications.

Ovens contain a cooking chamber which is lockable by a door and isdelimited by an oven muffle. During roasting and baking, the side wallsof the cooking chamber are soiled e.g. by splashing fat or meat juice orthe like. This soiling during baking and roasting cannot be prevented.For this reason, the manufacturers have proposed several ways to cleanthe walls, top and bottom, i.e. the inner space of the cooking chamber.One generally differentiates between catalytic and pyrolytic cleaning.

For pyrolytic cleaning, the cooking chamber has so-called grill rodswhich can be controlled and heated through a separate, electronicallycontrolled program and are preferably mounted to the top of the cookingchamber. The organic soiling is carbonized, i.e. completely burnt, attemperatures of more than 500° C. (Cepem Cie Euro Equip Menager[FR2605391] or Bosch Siemens Haushaltsgeräte GmbH [DE2526096]. Pyrolyticcleaning is demanding and expensive due to the required hightemperatures. Ovens with pyrolytic cleaning must have suitableprotective mechanism to block the door of the cooking chamber duringpyrolysis (from approximately 320° C., Bosch Siemens Hausgeräte GmbH [EP0940631] to protect the oven from improper operation. Since these ovensfurthermore require more expensive heating elements to be able tocontrol the high temperature at all, pyrolysis systems have beenestablished only in ovens of the top price bracket.

In view of the costs, the catalyst systems are preferable to pyrolysissystems since catalytic combustion of soiling takes place at lowertemperatures, i.e. below 500° C. Matsushita Elec. Ind. Co. Ltd.[JP03056144] proposes lining of the interior of the oven with acatalytically active coating which consists of a binder system and acatalytically active powder. Metal oxides are used as catalyst,preferably manganese dioxide and silicon resins are used as binders.This catalytic coating permits cleaning of the oven interior alreadybetween 380° C. and 400° C. as stated by the manufacturer. Mixture of acatalyst and a binder system or a layer matrix for coating the innersurface of an oven can also be found with other oven manufacturers.Toshiba [JP60147478] uses manganese oxide or ferrite as catalyst andsodium silicate as binder phase. In an analog fashion, Sharp KK[JP54135076] uses quartz sand or sodium silicate as binder phase andiron oxide or copper oxide as catalyst. These protective rights give nostatement about the effectivity of the two latter catalytic coatings.The onset temperature of the coating, i.e. the temperature at which thelayer starts to work, was reduced in accordance with the above-mentioneddocuments to 270° C. to 300° C. (Toshiba) and even to 250° C. (SharpKK). In practice this means, that there are catalytic coatings whichstart to break down fat etc. in the interior of the oven at temperaturesbelow 320° C., however, the efficiency of the coating is not sufficientto completely finish this decomposition. After each baking or roastingcycle, residues of non-decomposed fat remain in or on the layer liningthe interior of the oven, such that after a very short time, thefunction of the layer is impaired since it is varnished. For completedecomposition, these systems still require temperatures of usually morethan 380° C.

Finally, NGK Insulators Ltd [JP56095022] should be mentioned, which usemanganese oxide, copper oxide and iron oxide as catalysts, and a porousenamel as layer matrix to increase the amount of applied catalyst, aswell as the protective rights of Matsushita [JP02069574], Cie Euripeennepour L'Equ [FR2040822] and Hoover LtD [GB1177434] which all useflouropolymers as carrier layer for the catalysts to minimize thesurface energy of the carrier layer and prevent adhesion.

Pyrolytic cleaning is very effective at temperatures above 500° C. butis expensive due to the facts given by process technology. These systemsare currently used only for ovens of the top price bracket (maximally10% of all ovens). The reduction in cost promoted the development ofcatalytic cleaning. The inner walls of the cooking chamber are therebylined with a layer which always contains a catalyst. Suitable catalystsare manganese oxide, iron oxide and copper oxide, whereintemperature-resistant polymers, sodium silicate, quartz sand and enamelare used as binder phase of the catalyst or as layer component. Thecatalysts operate at temperatures of more than 380° C. which requiressafety measures producing additional costs. Only a few catalyticallyactive coatings are known whose onset temperature, i.e. start of fatdisintegration in the layer is between 250° C. and 350° C. In thesecases, large amounts of residues remain in or on the layer duringpermanent operation of the oven below 350° C. with the consequence thatthese oven inner coatings varnish very quickly.

Catalysis is subject to thermo-dynamic rules. A catalyst cannot changethe thermodynamics of a system but only lower the activation energy,i.e. the tendency to start the reaction. Although combustion of theorganic soiling occurs thermodynamically only at a higher temperature,it starts at a lower temperature if initiated by a catalyst. Not allparts of the organic soiling disintegrate at this low temperature, whichleaves residues which cause varnishing of the interior of the oven withthe consequence that the optic and haptic appearance of the oveninterior drastically deteriorates after only a few baking and roastingcycles.

It is the underlying purpose of the invention to develop a coating forthe interior of an oven which automatically eliminates the soilingproduced through roasting and baking, i.e. through application of atemperature of considerably less than 320° C., wherein the workingtemperature of the layer is preferably 250° C.

SUMMARY OF THE INVENTION

The object is achieved by a ceramic composition (mass), a mixture of aporous ceramic powder and an inorganic binder system, which comprises atleast one porous ceramic powder with a an average particle sizedistribution of more than 500 μm and an inorganic binder system whichcontains at least one nano-scale particle.

In this fashion, porous ceramic layers can be produced having hightemperature stability and abrasion resistance. These layers containlarge pores/pore volumes which are accessible for organic soiling (e.g.fats), and also small pores through the introduced porous ceramicparticles, which are not accessible for organic soiling. The porousceramic layers have a very high suction capacity and transport theorganic soiling (e.g. fat and meat juice) initially inside the inventivelayer. The soiling is spread, i.e. distributed on a very large surface.At a temperature of 250° C., the soiling is almost completelydisintegrated with no catalyst being contained in the layer. Precisematching of the binder system and the fact that at least onenanoparticle is used as binding phase, produces a very large innersurface, preferably larger than 20 m²/g, particularly preferred largerthan 70 m²/g and particularly preferred larger than 120 m²/g which isloaded with organic soiling. On the other hand, the reaction partneroxygen which is required for combustion, is stored already in the porousceramic parts, similar to a reservoir and is directly available suchthat the oxidative combustion of the soiling is initiated early andcarried out in quantity already at 250° C.

First-time production of a self-cleaning layer for ovens is achieved,which removes almost in quantity organic soiling at temperatures ofconsiderably less than 380° C., preferably considerably less than 320°C. A new possibility to clean ovens consists in the production of anactive ceramic layer without catalyst, which, however, offers thepossibility to spread organic soiling on a very large surface (due tothe nanoparticles) and provide the reaction partner, required foroxidation, in the form of a reservoir in the layer. Compared tocommercially available catalytic cleaning systems, the inventiveself-cleaning layer is moreover characterized by a considerably higherefficiency at lower temperatures, preferably between 280° C. and 250° C.preventing early varnishing of the coating.

The inventive ceramic layer is characterized by the presence of numerouspores of different sizes and a high inner pore volume. To generate thesepores, the inventive composition (mass) preferably contains twodifferent ceramic powder s particles and particularly preferred threedifferent ceramic powder particles. The ceramic particles used are, inparticular, chalcogenide, carbide or nitride powders, wherein at leastone of these powders is nano-scale. The chalcogenide powders may beoxide powder, sulfide powder, selenide powder or telluride powder,wherein oxide powder is preferred. Any powder which is conventionallyused for powder sintering, can be used. Examples are (optionallyhydrated) oxides such as ZnO, CeO₂, SnO₂, Al₂O₃, SiO₂, TiO₂, In₂O₃,ZrO₂, yttrium-stabilized ZrO₂, Fe₂O₃, Fe₃O₄, Cu₂O or WO₃ as well asphosphates, silicates, zirconates, aluminates and stannates, carbidessuch as WC, CdC₂ or SiC, nitrides such as BN, AIN, Si₃N₄, and Ti₃N₄corresponding mixed oxides such as metal-tin-oxides, e.g.indium-tin-oxide (ITO). Moreover, also mixtures of the stated powderparts can be used.

The inventive composition (mass) contains a ceramic powder which ischaracterized by a high specific, largely inner surface, larger than 50m²/g, preferably larger than 100 m²/g, and particularly preferred largerthan 150 m²/g. This porous ceramic powder has aan average particle sizedistribution with an average particle size of more than 500 nm,preferably larger than 1 μm and particularly preferred larger than 30μm. This ceramic powder is an oxide, hydroxide, chalcogenide, nitride orcarbide of Si, Al, B, Zn, Zr, Cd, Ti, Ce, Sn, In, La, Fe, Cu, Ta, Nb, V,Mo or W, particularly preferred of Si, Zr, Al, Fe and Ti. The use ofoxides is particularly preferred. Preferred inorganic solid particlesare aluminum oxide, boebmite, zircon oxide, iron oxide, siliconedioxide, titanium dioxide, silicates, stone powder, perlites andzeolites or mixtures of these inorganic solids.

The inventive composition moreover contains an inorganic binder system,which consists of a solvent and at least one nano-scale powder. Theprimary parts of the nano-scale powder may be present in an agglomeratedform, preferably in a non-agglomerated or substantially non-agglomeratedform. Any conventional solvent can be used preferably 2-butoxy ethanol,ethanol, 1-propanol, 2-propanol, particularly preferred water. Theceramic powder is an oxide, hydroxide, chalcogenide, nitride or carbideof Si, Al, B, Zn, Zr, Cd, Ti, Ce, Sn, In, La, Fe, Cu, Ta, Nb, V, Mo orW, particularly preferred of Si, Zr, Al, Fe, and Ti. The use of oxidesis particularly preferred. Preferred inorganic nano-scale solidparticles are aluminum oxide, boehmite, zircon oxide, iron oxide,silicon dioxide, titanium dioxide, and goethite or mixtures of theseinorganic nano-scale solids. To adjust the viscosity of the inorganicbinder system, all conventional inorganic and organic acids and lyes canbe used, preferably hydrochloric acid, phosphoric acid and nitric acid.

A third ceramic powder may be added to the inventive composition,optionally for precise adjustment of the porosity. This powder consistsof ceramic particles of an average particle size distribution of between10 nm and 1 μm, preferably between 150 nm and 600 nm. The substance ofthe third ceramic powder is oxide, hydroxide, chalcogenide, nitride orcarbide of Si, Al, B, Zn, Zr, Cd, Ti, Ce, Sn, In, La, Fe, Cu, Ta, Nb, V,Mo or W, particularly preferred of Si, Zr, Al, Fe and Ti. The use ofoxides is particularly preferred. Preferred inorganic solid particlesare aluminum oxide, boehmite, zircon oxide, iron oxide, silicon dioxide,titanium dioxide, silicates, and stone powder.

The inventive composition can optionally be extended through adding oneor more coloring inorganic components. Any conventional inorganiccolorants can be used as coloring component, preferably spinels. Thecombination of several coloring components permits arbitrary adjustmentof color effects (patterns and spots) in addition to pure colors.

The third optionally used ceramic powder is mixed with the likewiseoptionally used coloring powders and slurried with the solvent. Theporous ceramic powder and the inorganic binder system are added to thisslurry, thereby producing a ceramic suspension which can be applied,dried and subsequently compacted into a porous ceramic layer throughspin coating, dip coating, immersion, flooding or preferably sprayingonto a desired substrate. For compacting, temperatures of up to 1200° C.can be used, preferably between 400° C. and 1000° C. and particularlypreferred between 700° C. and 850° C.

The inventive ceramic composition permits application of porous, ceramiclayers onto metal, glass, enamel or ceramic surfaces having layerthicknesses of between 20 μm and 1 mm, preferably between 70 μm and 600μm.

In a particular embodiment of the invention, these porous, ceramiclayers can be covered with catalysts to permit utilization of theselayers for catalytic reactions, e.g. in the chemical industry.

The following example explains the invention without limiting it:

EXAMPLE 1

15.0 g of MARTOXID® (company Martinswerk) aluminum oxide powder is mixedwith 10.0 g of spinel pigment PK 3060 (company Ferro) and slurried with52.0 g water. 70.0 g of a porous aluminum oxide (NABALOX® NG100, companyNabaltec) is added thereby obtaining a highly viscous pasty slurry.Addition of 3.8 g of a 65% nitric acid greatly reduces the viscosityproducing a stirrable suspension. 26.38 g of an inorganic bindersolution (40% nano-scale zircon oxide/60% water) is added to thissuspension. The viscosity of the now sprayable suspension can bearbitrarily adjusted by small amounts of water and/or nitric acid.

The invention concerns a method for producing a highly porous, ceramiclayer and for applying this layer onto metallic, ceramic, enameledand/or glass substrates using porous ceramic particles, preferablyaluminum oxide, titanium oxide and zircon oxide and an inorganic bindersystem. The inorganic binder system contains at least one ceramic nanoparticle of a particle size of less than 100 nm, preferably less than 50nm and particularly preferred less than 25 nm. The solvent is water.Layers produced in this fashion are suited as self-cleaning activelayers, e.g. in ovens, in combustion engines etc. or generally forcoating substrates to drastically increase their specific surface, e.g.for catalyst substrates.

1. A method for producing a composition for a highly porous ceramiclayer, said method comprising: providing a porous ceramic powder havingan inner surface larger than 50 m²/g and an average particle sizedistribution of more than 500 nm; providing an inorganic binder systemcontaining at least one nano-scale powder and a solvent; and mixing theporous ceramic powder and the inorganic binder system to form thecomposition.
 2. The method according to claim 1 wherein said solventcomprises water.
 3. The method according to claim 1 wherein the porousceramic powder inner surface is larger than 100 m²/g and the averageparticle size distribution is more than 1 μm.
 4. The method according toclaim 1 wherein the porous ceramic powder inner surface is larger than150 m²/g and the average particle size distribution is more than 30 μm.5. The method according to claim 1 wherein the porous ceramic powder isselected from a group consisting of aluminum oxide, boehmite, zirconoxide, iron oxide, silicon dioxide, titanium dioxide, silicates, stonepowder, perlites, zeolites and mixtures thereof.
 6. The method accordingto claim 1 wherein said nano-scale powder is selected from a groupconsisting of Al₂O₃, AlO(OH), ZrO₂, TiO₂, SiO₂, Fe₃O₄, SnO₂ and mixturesthereof and wherein an average primary particle size of the nano-scalepowder is below 100 nm and the solvent comprises water or an alcoholselected from a group consisting of 2-butoxy ethanol, ethanol, and1-propanol and 2-propanol.
 7. The method according to claim 1 furthercomprising providing another ceramic powder, and wherein the anotherceramic powder is present in an amount providing adjustment ofcomposition porosity, said another ceramic powder having an averageparticle size distribution of between about 10 nm and 1 μm and saidanotherceramic powder is selected from a group consisting of aluminumoxide, boehmite, zircon oxide, iron oxide, silicon dioxide, titaniumdioxide, silicates and stone powder.
 8. The method according to claim 1wherein one or more inorganic colorants are added to the composition. 9.The method according to claim 1 further comprising providing a pluralityof inorganic colorants in amounts enabling adjustment of a color of thecomposition.
 10. The method according to claim 1 further comprisingapplying, the composition onto a ceramic, metallic, enameled or glasssubstrate through a step selected from a group consisting of spincoating, dip coating, immersion, flooding and spraying and furthercomprising drying and condensing the composition.
 11. The methodaccording to claim 10 wherein the composition is condensed at atemperature of up to 1200° C.
 12. A porous ceramic layer produced inaccordance with the method of one of the claims 1-5, claim 6, claim 7and claims 8-11.
 13. The method according to claim 10 further comprisingapplying the composition to an oven surface to act as a self-cleaninglayer.